The present invention relates to an electrooptic effect element and an electrical signal waveform measuring apparatus using the same and, more particularly, to improvement of an electrooptic effect element known as one of external optical modulators, and an electrical signal waveform measuring apparatus utilizing an E-O sampling (Electrooptic Sampling) system for measuring an electrical signal by light using the high-speed response characteristics of the improved electrooptic effect element and a technique for obtaining ultrashort light pulse having a pulse duration shorter than one picosecond.
In recent years, a high-speed electronic device has been remarkably improved In particular, a switching speed of an HBT (Heterojunction Bipolar Transistor) utilizing a compound semiconductor heterojunction at room temperature reaches 2.5 picosecond. In general, an electronic sampling oscilloscope is used as a measuring apparatus for estimating the switching speed of socalled ultrafast device However, the accuracy of this measuring apparatus is limited to a switching speed of a semiconductor device in a sampling head. Therefore, the capability of this measuring apparatus is insufficient to measure the speed of the ultrafast device, as a matter of course. For this reason, an attempt has been recently made to improve time resolution by employing a sampling head utilizing a Josephson junction (e.g., the Institute of Electronics and Communication Engineers of Japan, the 70th Anniversary National Meeting, Lecture Papers 266, 1989, Nakanishi et al.). According to the papers, it is inevitable that the structure of the measuring apparatus is complicated because a specific cryoelectronics (very low temperature technique) is required.
In the above situations, therefore, the following method is employed in many cases. That is, several tens of the ultrafast devices are connected to each other to constitute, e.g., a ring oscillator as a typical example, and a switching speed of each device is calculated based on the oscillating frequency of the ring oscillator (e.g., the 33th Joint Lecture associated with the Japan Society of Applied Physics, 1986 Spring, Papers 3p-t-8).
According to this method, although average characteristics of a large number of devices can be obtained, the characteristics of the individual devices, e.g., a leading waveform of a specific device cannot be observed.
An E-O sampling system utilizing light is proposed in place of the above method. This E-O sampling technique utilizing light is disclosed in the specification of an early U.S. Pat. No. 3,614,451 issued to Gunn in 1968. A measuring method of an electrical signal using a traveling wave electrooptic modulator utilizing an electrooptic effect element and ultrashort light pulse is disclosed in the above U.S.P. literature. In particular, according to this literature, the traveling directions of an electrical signal and the light pulse are inclined, so that the phase velocity of the electrical signal to be measured is equal to the group velocity of the light pulse in the traveling direction of the electrical signal, thus improving time resolution.
According to thus disclosed technique, however, a countermeasure against a change in output level of a light source of the ultrashort light pulse, temperature drift of the electrooptic effect element, or the like is not taken at all. Therefore, a high-precision E-O sampling system cannot be realized, and stable measurement cannot be performed. Thereafter, as a light source of the ultrashort light pulse has been improved and developed, this E-O sampling system has received a great deal of attention again as a waveform measuring system having a high time resolution. Several electrical signal measuring systems (PCT Publication No. 59-500186 and Japanese Patent Publication No. 60-253878, and U.S. Pat. Nos. 4,446,425 and 4,603,293) utilizing a Traveling-wave (TW) electrooptic effect element were disclosed by Mourou and Valdmanis.
The electrical signal measuring system by the E-O sampling method utilizing the above-mentioned conventionally known electrooptic effect element, however, has the following problems which have not been solved.
(1) The refractive index of a material for an electrooptic effect element is largely changed in accordance with a change in ambient temperature.
(2) When the electrooptic effect element is manufactured, it is difficult to obtain a preferable incident surface of a light beam.
(3) A change (drift) in output characteristics (e.g., an intensity or a wavelength) of ultrashort light pulse is inevitable.
(4) When an E-O sampling system is constituted by an optical system including a polarizer, the electrooptic effect element, an analyzer, and a photodetector, the perfect linear characteristics between the input and output characteristics of the E-O sampling system cannot be obtained.
(5) Temperature drift of the entire optical system and the photodetector are inevitable.
(6) In the entire system, it is difficult to set an operating point of the electrooptic effect element, and in particular, to perform fine adjustment of a compensator for compensating static birefringence.
(7) Since it takes about ten minutes to perform sampling for each waveform measurement, drift in the sampling time is inevitable.
(8) A sampling level need be corrected upon every replacement operations of a light pulse source.
As a result, the above problems (1) to (6) undesirably cause degradation of the E-O sampling system. The above problems (5) to (8) undesirably cause unstable measurement.